Bonsai is usually a term for the extreme end of techniques to keep plants small. If someone restricts a plant's growth to half its natural size but no where near the smallest size possible, is that plant a bonsai? Is it half bonsai? From the Chinese penzai, bonsai is a Japanese word where the prefix bon means a tray-like pot. So, the word itself implies that the plant would need to be in a tray-like bonsai pot for the term to be accurate. While many people will call a plant "bonsai" if it's a potted miniature version of a much larger plant, I think the very shallow container and corresponding root restriction/trimming is what bonsai is all about, regardless of whether the plant could be kept smaller.
We keep our moms in about 3qts of soil and they stay around 6-7" tall with proper care. It gets harder to keep them small the older they are, so they are replaced by clones every couple years or so.